The functioning of the nervous system is determined to a large extent by the intricate network of connections between neural cells. A necessary step in the development of this network is the guidance of axons to their appropriate targets during embryogenesis. The long-term aim of this proposal is to understand the cellular events and molecular mechanisms used to guide axons to their targets in the mammalian nervous system. Axons are thought to be guided by both short-range cues and long-range diffusible chemotropic factors, but our understanding of chemotropic mechanisms has been limited by the lack of identification of any endogenous chemotropic factor. Recently, a family of endogenous axonal chemoattractants, the netrins, has been isolated. The identification of these factors provides an opportunity to gain a molecular understanding of chemotropic guidance mechanisms and to determine the contribution of chemoattractants to the guidance of specific classes of axons in the developing embryo. The properties and expression patterns of the two known netrins, F- and S-netrin, suggest that they contribute to guiding the axons of a set of neurons in the spinal cord (commissural neurons) to one of the targets of these axons, the floor plate of the spinal cord. F-netrin might attract them from a distance, while S-netrin might act more locally. Several approaches will be taken to test these possibilities. The forms and distributions of the netrins encountered by commissural axons enroute to the floor plate will be determined, to understand how chemotropic factors are presented to axons in order to effect guidance. The full range of biological activities of the netrins -- and potential differences between the two netrins -- will be explored, in an attempt to understand why two different netrins are apparently being used to guide the axons. The portions of the molecules responsible for mediating their effects on commissural axons will be identified. Finally, to determine the actual contribution of the netrins to commissural axon guidance in the embryo, mutant mice will be created in which the netrin genes have been inactivated, and examined for defects in the guidance of these axons to the floor plate. These studies will provide insight into the mechanisms by which chemotropic factors guide axons, and the neurological consequences of abnormal function in these particular attractants. Defects in the initial establishment of neural connections may have devastating consequences on the eventual functioning of the nervous system, and may account for a variety of psychiatric and neurological disorders of childhood. Understanding the mechanisms involved in the normal establishment of connections, and the consequences of abnormal functioning of these mechanisms may therefore provide insight into some of these disorders and the means to treat them.